1. Field of the Invention
The present invention relates generally to mobile platform communications systems, and particularly to a vehicle or aircraft-based beam steering method and apparatus.
2. Description of Related Art
As the convergence of voice, data and video signals becomes more commonplace, payloads mounted on low earth orbit (LEO) or medium earth orbit (MEO) vehicle platforms and aviation platforms are being increasingly used to provide wireless communication links for surface-based broadband communication devices. A payload so utilized receives data packets transmitted from surface-based devices, with each data packet including a downlink address header corresponding to a surface location below. The payload modulates each data packet onto an electromagnetic communication beam transmitted via a directional antenna to the designated downlink address, thereby effecting a communication downlink.
As the platform moves along its designated orbital path, the payload must steer its modulated beams to account for platform movement and to maintain coverage within the mobile platform""s antenna field of view. Conventional payload-based beam steering systems generate beam angle data sets on the fly, i.e., in real-time, for each downlink data packet received by the payload. In this processing, the downlink address in each packet header is mapped to a specific angle data set that provides the optimum beam-pointing angle for communicating that packet of information to the designated downlink address. The data set is then utilized to steer the corresponding modulated beam to ensure accurate transmission of the particular data packet to the downlink address.
However, the real-time beam steering systems of the type described above have certain limitations. For example, because payloads link high bandwidth systems, and because an angle data set must be generated for each data packet received by the payload, the real-time beam steering approach is computationally intensive, and thus overall payload hardware and power requirements are significant. For example, payload modulation of 16 beams during a 2 xcexcs burst period would require beam steering data to be calculated at a rate of 8 million data sets per second.
In addition, the high computational processing required for real-time beam steering systems typically limit the ability to compensate for variables such as earth oblateness, RF beam profile variances, vehicle pitch, yaw and roll deviations, orbit eccentricities, electrical and mechanical antenna pointing offsets, downlink target elevation and beam radiation pattern errors. Such variables contribute to a generalized composite error function that tends to increase in proportion to the nadir angle, which is the angle between a nadir vector (a vector formed between the vehicle and a surface point directly below the vehicle relative to the Earth""s center) and the beam-pointing vector. Therefore, the accuracy of the resulting data sets used for beam steering may be limited to a certain degree, particularly when a downlink address is located near an outer limit of the antenna radiating field of view and therefore creates a large nadir angle.